docs: def com_google_fonts_check_negative_advance_width(ttFont):
    """ Check that advance widths cannot be inferred as negative. """
    failed = False
    for glyphName in ttFont["glyf"].glyphs:
      coords = ttFont["glyf"][glyphName].coordinates
      rightX = coords[-3][0]
      leftX = coords[-4][0]
      advwidth = rightX - leftX
      if advwidth < 0:
        failed = True
        yield FAIL, ("glyph '{}' has bad coordinates on the glyf table,"
                     " which may lead to the advance width to be"
                     " interpreted as a negative"
                     " value ({}).").format(glyphName,
                                            advwidth)
    if not failed:
      yield PASS, "The x-coordinates of all glyphs look good."